1. Field of the Invention
The present invention generally relates to safety systems for buildings, and in particular to a building safety system which shuts off the supply of gas to a building in the event of any one or more of a gas leak, a fire or an earthquake, and is also useful for the detection from ships of undersea earthquakes.
2. Summary of the Prior Art
Recent major earthquakes have demonstrated that the damage sustained by buildings results from two primary sources, namely the ground accelerations induced by the earthquake itself, and subsequent explosions and fires caused by flammable gas leaking from damaged pipe lines. Many older or poorly-built buildings are destroyed by the ground accelerations. On the other hand, modern buildings, which are properly designed and built in accordance with accepted engineering practice for earthquake-prone regions, typically withstand moderate ground accelerations with comparatively little structural damage. In either type of building, gas pipelines within the building can be damaged or ruptured by the ground accelerations, creating an extreme explosion/fire hazard. This hazard is magnified by the likelihood that water-mains may be severed, hampering firefighting efforts. Furthermore, electrical power may also be interrupted, thereby preventing other safety systems from functioning properly, even if these safety systems have not been physically damaged by the earthquake.
According to the California earthquake standards, the degree of hazard posed by ground accelerations is dependent on both the magnitude and frequency of the acceleration. This is illustrated in FIG. 7, which shows the earthquake hazard posed by accelerations at frequencies between 0 and 10 Hz. Ground accelerations falling above the threshold acceleration/frequency curve in FIG. 7 are considered to pose an earthquake hazard and thus should trigger an alarm. Conversely, ground accelerations falling below the threshold acceleration/frequency curve in FIG. 7 are considered to be safe and thus should not trigger an alarm. For example, ground accelerations of 0.3 g (i.e. 0.3xc3x979.81=2.94 m.secxe2x88x922) or greater at 2.5 Hz are considered to be an earthquake hazard. Accelerations of 0.08 g at 1.0 Hz and 2.5 Hz are considered safe, as are accelerations of 0.4 g at 10 Hz. Accordingly, an earthquake detector must be calibrated to be triggered by ground accelerations of greater than 0.3 g at 2.5 Hz. However, in order to avoid false-alarms, such as by passing vehicles, the detector must not be triggered by acceleration levels which are in the safe zone of the graph of FIG. 7, even if the detected acceleration magnitude is higher than 0.3 g.
U.S. Pat. No. 5,101,195 (Caillat et al.) discloses a motion detector for detecting an earthquake in order to automatically shut off gas supplies to a building in the event of an earthquake. The detector of Caillat et al. includes a highly damped cantilever beam arranged to generate an electrical signal as the end of the beam moves up and down. The signal generated by the moving beam is passed through a band-pass filter, which passes signal frequencies between 3 Hz and 14 Hz. The filtered signal is then passed to a sensor circuit, which produces an alarm signal. It will be noted that the detector of Caillat et al. attenuates low-frequency vibrations, and as such would be substantially incapable of detecting low-frequency earthquake accelerations i.e. at 2.5 Hz, which, as discussed above, are considered by the California earthquake standards to be hazardous.
An object of the present invention is to provide a reliable building safety system for shutting off a supply of gas to a building in the event of an earthquake.
Another object of the present invention is to provide a building safety system capable of shutting off a supply of gas to a building in the event of a fire or gas leak.
Thus the present invention provides a safety system for a building. The safety system includes an earthquake detector circuit for detecting ground accelerations exceeding a threshold acceleration/frequency curve dividing safe and hazardous ground accelerations throughout a predetermined frequency range. The earthquake detector includes an accelerometer for measuring linear acceleration along an axis, and for generating an acceleration signal indicative of a magnitude of the measured acceleration. A filter circuit produces a filtered acceleration signal from the acceleration signal. The gain/frequency characteristic of the filter circuit is a reflection of the threshold acceleration/frequency curve, such that ground accelerations falling on the threshold acceleration/frequency curve yield a filtered acceleration signal having a substantially constant value throughout the frequency range. Finally, a threshold circuit compares the filtered acceleration signal to a predetermined threshold voltage, and produces an earthquake detected signal indicative of the comparison result.
In a preferred embodiment of the present invention, a main unit including a microprocessor is responsive to the earthquake detected signal, and generates a first control signal in response to the earthquake detected signal. A valve controller can suitably be provided to close a gas valve to shut off a supply of gas to the building in response to the first control signal.
An embodiment of the present invention also includes a gas detector circuit capable of detecting a gas in air, and generating a gas detected signal in response to detection of the gas. In this case, the main unit is also made responsive to the gas detected signal, so as to generate the first control signal in response to the gas detected signal.
Preferably, the gas detector circuit includes both a gas detect portion and a trouble detect portion, so that the gas detector circuit is capable of detecting faulty operation of the gas detector portion, as well as detecting gases in air.
An embodiment of the present invention also includes a smoke detector circuit for generating a fire detected signal. In this case, the main unit is also made responsive to the fire detected signal, so as to generate the first control signal in response to the fire detected signal.
In another embodiment of the present invention, the main unit includes a gas detector combine circuit, which combines the signals of two or more independent gas detector circuits and produces a single gas detector signal line. By this means, a plurality of gas detector circuits can be used, while retaining a single input line to the microprocessor. The gas detector combine circuit preferably includes respective gas detect and trouble detect portions for combining respective gas detected signals and trouble detected signals generated by the gas detectors.